The present invention relates to a semiconductor radiation detector suitable for a device for irradiating a specimen with a charged particle such as an electron beam and x-ray, detecting characteristic x-ray generated from the specimen and analyzing the elements of the specimen and a device for irradiating a specimen with x-ray, detecting x-ray which is transmitted through or reflected from the specimen and analyzing the structure of the specimen, the manufacturing method and an apparatus of radiation detection using the above semiconductor radiation detector.
A method of irradiating a specimen with a charged particle such as an electron beam or x-ray, detecting characteristic x-ray generated from the specimen or fluorescent x-ray and analyzing the specimen is known. For its typical example, a method called energy dispersive x-ray spectroscopy of irradiating a specimen with an electron beam in an electron microscope, detecting characteristic x-ray generated from the specimen and analyzing the elements of the specimen can be given.
As characteristic x-ray or fluorescent x-ray has energy peculiar to an element composing a specimen, the number of the generation of x-ray per unit time to analyze elements is required to be counted every energy of x-ray. The energy dispersive x-ray spectroscopy is a method of using a detector wherein an output signal having height proportional to the energy of incident x-ray is acquired, identifying the energy of x-ray by combining the detector with a pulse-height analyzing circuit and analyzing elements.
For an x-ray detector used for the above energy dispersive x-ray spectroscopy, there is a semiconductor radiation detector (hereinafter simply called a detector) using a semiconductor crystal such as silicon and germanium.
An apparatus using these detectors and having energy resolution of approximately 140 eV for x-ray having energy of 5.9 keV is known. For the structure of a semiconductor radiation detector, three types of a pin-type, a pn-type and Schottky-barrier type (or a surface-barrier type) are known.
For a detector according to this type of energy dispersion method heretofore used, there is a detector produced by dispersing lithium in a silicon crystal called a lithium-drifted silicon detector.
Of the above three types of detectors, for a first example, the typical contour of a pin-type detector is shown in FIG. 7A. FIG. 7A shows a cross section viewed along a line A-Axe2x80x2 in FIG. 7B showing the appearance of the detector. The detector uses a p-type silicon crystal 101 and the outline is cylindrical and the detector has a concentric deep groove 6.
The pin-type has structure in which an intrinsic semiconductor region (an i layer) 1 formed by dispersing lithium in a semiconductor substrate 101 is held between a p-type layer 2 and an n-type layer 3 respectively formed on opposite surfaces, gold is respectively deposited on the surfaces of the p-type layer 2 and the n-type layer 3 and electrodes 4 and 5 are formed.
Negative voltage is applied to the electrode 4 on the p-type side of the detector by a bias supply 50 (reverse bias voltage is applied). Normally, x-ray is made incident from the surface of the electrode 4 on the p-type side. When x-ray 10 is incident upon the intrinsic semiconductor region 1, a secondary electron is generated and produces an electron-hole pair 20 and 21, losing energy. The generated electron 20 moves to the electrode 5 on the n-type side by an electric field between the electrodes 4 and 5.
The number of the generated electron-hole pairs is proportional to the energy of incident x-ray. The electron 20 which reaches the electrode 5 is converted to a voltage pulse 52 having height proportional to the number by an amplifier 51 and the energy of x-ray is identified by a pulse-height analyzer 53.
Reverse bias voltage applied to the electrodes 4 and 5 is set to high voltage of approximately 1000 V to prevent an electric charge (the electron-hole pair 20 and 21) generated in the intrinsic semiconductor region 1 from being recombined and from being annihilated.
To acquire high energy resolution for the ability of a detector, it is required to reduce leakage current which flows in a detector when reverse bias voltage is applied down to 100 fA or less and to reduce the capacitance of the detector. Therefore, a detector is housed in a vacuum container, is cooled by liquid nitrogen and others, leakage current thermicly generated is reduced by keeping the detector at low temperature and further, surface leakage current is reduced by the concentric deep groove 6.
The capacitance of a detector is in inverse proportion to the thickness of the intrinsic semiconductor region 1 and is proportional to area S. The area S means the cross section of a part (the intrinsic semiconductor region 1) inside each groove 6 and is a sensitive part to x-ray. The thickness of the intrinsic semiconductor region 1 is set to approximately 3 to 5 mm.
In the case of a silicon detector, characteristic x-ray having energy of approximately 20 keV with the above thickness can be detected efficiently. For area S, a silicon detector having area of 10 to 30 mm2 is known. If area is further large, capacitance is increased and energy resolution required for analyzing elements is not acquired. If area is 20 mm2, that is, the diameter inside each groove 6 is approximately 5 mm, a silicon detector having the outside diameter of approximately 11 mm is known.
Next, in a pn-type detector for a second example, an n-type layer or a p-type layer of high density is formed on the surface including a p-type or an n-type semiconductor crystal in place of the above intrinsic semiconductor region 1 to produce pn junction and the above detector utilizes a depletion layer generated by applying voltage in a reverse direction. The high-density same-type layer is formed on each opposite surface and further, each electrode is formed on the layer.
When x-ray is incident on the depletion layer in a state in which reverse bias voltage is applied between these both electrodes and the depletion layer is generated in pn junction, an electron-hole pair 20 and 21 are generated and the electron 20 moves on the side of the electrode 5 by an electric field generated in the depletion layer as in the intrinsic semiconductor region 1 of the pin-type detector shown in FIG. 7A.
Also, for a third example, a detector utilizing a depletion layer generated by applying voltage to Schottky barrier formed by forming a metal electrode on the surface of a semiconductor substrate by gold and others in a reverse direction is called Schottky-barrier type detector (or a surface-barrier type detector). The thickness of a depletion layer is proportional to the square root of applied voltage and is in inverse proportion to the square root of the density of impurities in a crystal. To acquire a depletion layer 3 mm thick by applied voltage of 1000 V, a crystal of higher purity by 3 or 4 digits is required, compared with a crystal used for producing a normal transistor and a normal integrated circuit.
As for Schottky-barrier type detector, to acquire a depletion layer 3 mm thick by applied voltage of 1200 V as an example of the numerical value, a crystal of the purity of approximately 5xc3x971011 pieces per 1 cm3 in the density of impurities is required. As recent crystal manufacturing technology is developed, a crystal of high purity which meets the above specification can be manufactured and is actually utilized.
As for a heretofore used lithium-drifted silicon detector, as lithium is thermally diffused when the detector is left at room temperature for a long time and has a bad effect upon the characteristics of the detector such as capacitance increases, the detector is said to be always kept at low temperature, however, each type detector shown in these first to third examples using a high-purity crystal is not required to be always kept at low temperature.
For the contour of a detector, a detector having a contour in which the thickness of a periphery outside a groove is thinned and shown in FIGS. 8A and 8B and a detector provided with a cylindrical brim and shown in FIGS. 9A and 9B are known as disclosed in the U.S. Pat. No. 5,268,578 except the contour provided with the deep groove 6 shown in FIG. 7A. There is effect that leakage current is reduced in both contours. These groove and brim are formed by machining with a supersonic wave and further chemical etching of the machined surface.
Further, as disclosed in Japanese Patent Laid-Open No. 9-92868 formerly proposed by these inventors, there is a polygonal detector manufactured by a process shown in FIG. 10 wherein leakage current is reduced by adopting a mirror finished and polished surface.
The outline of each process for manufacturing the above detector according to a manufacturing process shown in FIG. 10 will be described below.
First, there is a process for dicing a rectangular parallelepipedic specimen (crystal) for example from a wafer to produce the detector.
Next, there are a process for polishing the surface of the crystal a mirror finished surface and a process for removing a minute damage due to polishing by etching.
Next, there is a process for doping n-type or p-type impurities in the crystal by ion implantation and annealing it and there is a process for removing an oxide film (a natural oxide film) on the surface of the crystal, depositing metal for forming an electrode on the surface and on the back of the specimen and forming a diode.
Finally, there is a process for forming an insulating passivation film on the whole specimen except the surface of the electrode after the surface of the specimen is cleaned.
The outside drawing of an example (a pin-type detector) of a detector acquired as described above is shown in FIG. 11B and a cross section viewed along a line A-Axe2x80x2 in FIG. 11B is shown in FIG. 11A. As shown in FIG. 11, a reference number 102 denotes a high-purity n-type silicon crystal, 2 denotes a p-type layer, 3 denotes an n-type layer, 4 and 5 denote an electrode and 7 denotes a polyimide passivating film.
For the other documents related to the formation of the passivating film 7, Japanese Patent Laid-Open No. 9-36410 for example can be given. In the case of the example, a passivating film is also formed in a final manufacturing process after an electrode is formed on the surface and on the back of a detector. That is, as shown in FIG. 12, in a final manufacturing process after a common electrode 4 and individual electrodes 5 are formed on the surface and on the back of a detector 100, the side (the end face) 8 of the detector 100 and the periphery 9 except the main surface which functions as the light receiving surface of the above common electrode 4 are covered with an insulating film (an oxide film) in the shape of a frame. In this example, a solder bump 5a is formed on the surface of the individual electrode 5 on the back.
However, as for two structures described in the first and second examples of the above detector, there are problems that a semiconductor crystal is required to be cut cylindrically as shown in FIG. 7A, a deep groove is required to be formed, difference in a level is required to be made on the side on which a groove is formed as shown in FIG. 8A, control in the dimension of these grooves and the control of a surface state of the groove are difficult, a manufacturing yield is bad, work is complex and a product is high-priced.
Also, further, there are problems that if plural detectors are arranged to enhance sensitivity in detecting x-ray, it is difficult to enhance mounting density because the cross section of each detector is circular and the area of a part sensitive to x-ray is small, compared with the outer cross section of the detector because ineffective areas such as a groove and a brim are provided. The above area is hereinafter called effective area. Further, a detector the cross section of which is not circular is also known, however, there is a problem that the machining of a groove and a brim is more complicated.
Also, as the process flowchart as prior art shown in FIG. 10 and the pin-type detector shown in FIGS. 11A and 11B, a polygonal detector adopting mirror polishing is characterized in that as the contour is simple, the manufacture is simple and a large effective area ratio is obtained. However, the yield of a product where leakage current 100 fA or less is generated by the application of reverse bias voltage of 1000 V is at most approximately 2%. Particularly, the yield in the case of Schottky-barrier type detector described as the above third example is low. In most Schottky-barrier type detectors, the typical value of voltage at which leakage current rapidly increases, breakdown starting voltage is approximately 200 V. Therefore, there is a problem that it is difficult to manufacture an actually usable detector.
When this type of detector is manufactured by prior art, a manufacturing method of forming a passivating film in a required part of the detector as a final process after a diode is completed via a process for diffusing impurities and a process for forming an electrode respectively required for a detector has been customarily adopted as to any detector of a pin type, a pn type and Schottky-barrier type.
Also, when this type of detector is manufactured, there is a problem that the following process is troublesome because an impurities diffusing process is executed every semiconductor crystal diced from a wafer.
Therefore, a first object of the present invention is to enable setting voltage at which leakage current rapidly increases, breakdown starting voltage to a still higher value than conventional 200 V and to provide a reliable semiconductor radiation detector.
A second object is to provide a manufacturing method of an improved semiconductor radiation detector wherein a detector the leakage current of which is 100 fA or less is acquired at the yield of at least 90% even if reverse bias voltage of at least 1000 V is applied.
A third object is to provide an apparatus of radiation detection provided with a semiconductor radiation detector the leakage current of which is 100 fA or less even if reverse bias voltage of at least 1000 V is applied.
The above first object is achieved by a semiconductor radiation detector characterized in that an insulating passivation film covers the side of a crystal and is provided except a main part of an electrode formed region formed on the surface and on the back of the semiconductor crystal and in the vicinity of the periphery of the main part so that the insulating passivation film surrounds the above periphery, at least an electrode on the side of a light receiving part is expanded from the main part of the electrode formed region of the semiconductor crystal so that the above electrode covers the periphery of the insulating passivation film and both peripheries are overlapped and laminated based upon a semiconductor radiation detector provided with diode structure configured by an electrode respectively provided on the surface and on the back of a semiconductor crystal, an insulating passivation film covering at least the side of the crystal of the above diode structure, a light receiving part configuring the main part of the surface electrode in the diode structure and a signal output part for outputting a signal generated in the diode structure from the electrode on the back as a detection signal when a radiation is incident on the light receiving part in a state in which reverse bias is applied between both electrodes.
As for the configuration of the detector according to the present invention, any radiation detector of a pin type, a pn type and Schottky-barrier type is effective, however, particularly Schottky-barrier type detector is the most effective.
A semiconductor crystal is represented by silicon, however, a compound semiconductor crystal may be also used and generally, a well-known semiconductor crystal used for a detector can be used.
As for an insulating passivation film, an organic insulating film formed by polyimide and others is desirable because the organic insulating film is easy to form its pattern, however, an inorganic insulating passivation film may be also formed by SiO2 and others by well-known chemical vapor deposition (CVD).
The detector according to the present invention which is the second object can be easily acquired by a manufacturing method described below.
A process in the manufacturing method according to the present invention is characterized in that first, after both surfaces of a wafer are polished, impurities required for the configuration of the detector are doped inside the wafer after a process for removing a damaged layer caused by a cleaning process and polishing on the wafer and removing a natural oxide film formed on the surface of the wafer by cleaning using hydrofluoric acid and others. The above process is also characterized in that a crystal in size required for forming the detector from the wafer into which the impurities are doped is diced in the shape of a cube for example for a later detector manufacturing process.
Hereby, multiple crystal specimens for forming the detector into which the required impurities are doped under the same condition can be prepared at a time.
Heretofore, the doping of impurities into a wafer has been individually executed for a diced specimen after the wafer is diced into a crystal in size required for forming the detector.
Second, the process in the manufacturing method according to the present invention is characterized in that the timing (the order) of the process for forming an electrode on the surface and on the back of a crystal specimen for forming the detector into which impurities are doped is shifted to a final process after the process for forming the insulating passivation film and electrode structure on the side of the light receiving part is improved.
As described above, heretofore, after an electrode is formed on a crystal specimen for forming the detector into which impurities are doped beforehand, the insulating passivation film is customarily formed on the surface of the detector except the electrode as a final process. However, the voltage at which leakage current generated when reverse bias voltage is applied rapidly increases (breakdown starting voltage) of the detector manufactured according to the above process is 200 V and is low.
These inventors performed various experiments and examined them to enable setting the above breakdown starting voltage to a higher value than 200 V. As a result, unexpectedly when the above process for forming the electrode is shifted after the process for forming the insulating passivation film to be a final process of manufacturing the detector and electrode structure on the side of the light receiving part is improved as described later, the above breakdown starting voltage is remarkably enhanced and unexpected characteristics exceeding conventional 200 V by far are acquired.
Also, a fact that a detector the leakage current of which is 100 fA or less even if reverse bias voltage of at least 1000 V is applied can be acquired at the yield of at least 90% becomes apparent.
The process for forming the insulating passivation film and the process for forming the electrode according to the present invention will be described in detail below.
To explain the process for forming the insulating passivation film, first, a crystal specimen used for this process is prepared via the following preparation process. That is, at least the side of a crystal specimen diced from a wafer for forming the detector is polished a mirror finished surface and further, etching processing is executed to remove a damaged layer by the above polishing.
In a process for polishing to be a mirror finished surface, in the case of Schottky-barrier type detector, a layer where impurities are diffused on the back of a specimen is left without being polished, the side is polished and a layer where impurities are diffused on the surface to be the light receiving part is also polished and is removed. If a pin-type detector and a pn-type detector are formed, only the side is polished and both the front surface and the back on which an electrode is formed are not polished.
In an etching process, an electrode formed region on the surface and on the back is covered with a resist film if necessary. That is, in the case of Schottky-barrier type detector, a resist film (a photoresist film) is formed in only an electrode formed region on the back of a specimen and the other surface is etched, protecting an impurity diffusion layer. If a pin-type detector and a pn-type detector are formed, only the side is etched, protecting an electrode formed region both on the surface and on the back with a resist film.
The specimen to which etching processing is applied as described above is cleaned by an organic solvent if necessary, further, is cleaned by inorganic aqueous solution such as hydrofluoric acid and the surface of the specimen is cleaned. If a resist film is used in etching processing, the specimen is cleaned by an organic solvent to remove a residue of an organic matter. A natural oxide film generated on the surface of the specimen is removed by cleaning using inorganic acid aqueous solution.
An insulating passivation film is formed on the surface of the specimen the surface of which is cleaned via the above processing. The insulating passivation film is continuously provided from the whole area of the side of the specimen to the periphery close to the side except the main part of the electrode formed region formed both on the surface and on the back of the specimen.
For the insulating passivation film, an organic insulating film formed by polyimide resin and polyamide resin is desirable and can be easily formed by applying resin solution to a specimen. Besides, an inorganic insulating passivation film such as SiO2 may be also formed by well-known CVD.
Next, the process for forming an electrode will be described. An electrode is respectively formed on the front surface and on the back of the above specimen on which the insulating passivation film is formed as a final manufacturing process. The electrode on the side of the light receiving part is laminated so that the above electrode on the side of the light receiving part covers from the electrode formed region which is surrounded by the insulating passivation film on the surface of the specimen and inside which a crystal is exposed (the main part of the electrode formed region) to the periphery of the insulating passivation film.
As described above, in the present invention, structure in which the periphery of the electrode on the side of the light receiving part covers the periphery of the insulating passivation film and both peripheries are overlapped is guessed one of reasons why excellent characteristics that electrostatic focusing is relaxed and thereby, breakdown starting voltage exceeds conventional 200 V by far are acquired.
As the surface of a specimen is cleaned and an electrode is formed in a state in which the insulating passivation film is formed, the following particulate adheres to the insulating passivation film and does not directly adhere to the surface of a crystal directly and even if a particulate of electrode material is dispersed on the side of the specimen in the process for forming an electrode and adheres and as a result, it is guessed that the deterioration of the characteristics is remarkably reduced.
According to the manufacturing method, as the insulating passivation film is previously formed and an electrode is formed afterward, it is easy to cover the periphery of the insulating passivation film with the periphery of the electrode on the side of the light receiving part and the degree of the overlap of both peripheries can be arbitrarily controlled by arbitrarily selecting the size of an electrode pattern.
It is desirable that an electrode on the back of a specimen also has the same contour as the electrode on the side of the light receiving part, however, as heretofore, the periphery of the insulating passivation film and the periphery of the electrode may be also close in place of laminated structure that the periphery of the insulating passivation film is covered.
These electrodes can be easily formed by well-known film formation technology such as deposition, sputtering and CVD and pattern formation technology by lithography.
For electrode material, at least one kind of gold, palladium and aluminum is used for desirable material.
The third object of the present invention is achieved by applying the semiconductor radiation detector acquired as described above to an apparatus of radiation detection having well-known configuration. That is, as the above apparatus of radiation detection is provided with the semiconductor radiation detector the leakage current of which is 100 fA or less even if reverse bias voltage of at least 1000 V is applied, a reliable apparatus of radiation detection which is sensitive in radiation detection can be realized.
The present invention is made based upon the above experiments and the details of the contents of the experiments will be described in embodiments.